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Background and purpose: Mitochondrial diseases (MDs) are heterogeneous disorders caused by mutations in nuclear DNA (nDNA) or mitochondrial DNA (mtDNA) associated with specific syndromes. However, especially in childhood, patients often display heterogeneity. Several reports on the biochemical and molecular profiles in children have been published, but studies tend not to differentiate between mtDNA-and nDNA-associated diseases, and focus is often on a specific phenotype. Thus, large cohort studies specifically focusing on mtDNA defects in the pediatric population are lacking. Methods:We reviewed the clinical, metabolic, biochemical, and neuroimaging data of 150 patients with MDs due to mtDNA alterations collected at our neurological institute over the past 20 years.Results: mtDNA impairment is less frequent than nDNA impairment in pediatric MDs.Ocular involvement is extremely frequent in our cohort, as is classical Leber hereditary optic neuropathy, especially with onset before 12 years of age. Extraneurological manifestations and isolated myopathy appear to be rare, unlike adult phenotypes. Deep gray matter involvement, early disease onset, and specific phenotypes, such as Pearson syndrome and Leigh syndrome, represent unfavorable prognostic factors. Phenotypes related to single large scale mtDNA deletions appear to be very frequent in the pediatric population. Furthermore, we report for the first time an mtDNA pathogenic variant associated with cavitating leukodystrophy. Conclusions:We report on a large cohort of pediatric patients with mtDNA defects, adding new data on the phenotypical characterization of mtDNA defects and suggestions for diagnostic workup and therapeutic approach.